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Metal-Rich PNe in the Outer Reaches of M31

Spectroscopic data of two relatively [O III]-luminous-PNe have been obtained with the 10.4-m Gran Telescopio Canarias. M174 and M2496 are each ~1 degree from the center of M31 along opposite sides of its minor axis. The ensemble of these two distant PNe plus 16 similarly luminous outer-disk PNe published earlier (Kwitter et al. 2012 & 2013) forms a homogeneous group in luminosity, metal content, progenitor mass, age, and kinematics. The main factual findings of our work are: (1) O/H (and other low-mass alpha elements and their ratios to O) is uniformly solar-like in all 18 PNe <12+log(O/H)> = 8.62 +/- 0.14); (2) the general sky distribution and kinematics of the ensemble much more closely resemble the rotation pattern of the classical disk of M31 than its halo or bulge; (3) the O/H gradient is surprisingly flat beyond R_g ~30 kpc, and may be flat throughout the entire range of R_g covered in the full study. The PNe are too metal-rich to be bona fide members of M31's disk or halo, and (4) the abundance patterns of the sample are distinct from those in the spiral galaxies M33, M81, and NGC 300. Using standard PN age diagnostic methods (which are readily challengeable) we suggest that all of the PNe formed -2 GY ago in a starburst of metal-rich ISM that followed an M31-M33 encounter about 3 GY ago. We review supporting evidence from stellar studies. Other more prosaic explanations, such as dwarf galaxy assimilation, are unlikely.